1. Field of the Invention
This invention relates to deposition of metal-silicide layers, and more specifically, to low temperature deposition of titanium-silicide layers using plasma-enhanced chemical vapor deposition.
2. Description of the Prior Art
In the past, integrated circuit devices have made extensive use of polysilicon layers, both as self-aligned gates for field-effect transistors and for electrical interconnects between various components. Traditionally, deposited polycrystalline silicon has been doped with conventional donor or acceptor elements to reduce its resistivity into the one milliohm-centimeter range. Since the film thickness is limited to about 0.5 microns maximum to permit patterning and reasonable step coverage, sheet resistances less than about 20 ohms per square are difficult to obtain. Such a value is too high for complex, high-performance integrated circuits because ohmic drops become too high. As a result, much effort has been devoted to the investigation of refractory metal silicides which have volume resistivities in the range 10-100 microohm-centimeters. Such values could enable sheet-resistances in the range of one ohm per square, enabling significant performance enhancement in integrated circuit applications.
While silicides of titanium, tantalum, tungsten, and molybdenum have been prepared by a number of techniques and have exhibited low resistivities, reproducibility of both resistivity and thickness has been poor, films have been hard to pattern, and the layer has been prone to peeling under thermal excursions. It is known to vacuum co-deposit silicon and a refractory metal and then react the mixture to form the metal silicide. Among other problems, a high temperature is usually required; this is undesirable from the standpoint of junction movement in shallow junction devices. Similarly, it is known to deposit refractory metal silicides from gaseous reactant species comprising silicon and a suitable refractory metal. Again, high temperatures may be required to get reaction, and since the gaseous species typically decompose at different temperatures, it is extremely difficult to get uniform deposition over a large number of work pieces. Furthermore, it is known to deposit a metal such as titanium onto various types of substrates using a plasma enhanced chemical vapor deposition system, however, in, for example, the publication entitled Titanium Deposition Onto Copper Substrates Using the Cold-Plasma Technique by Ruder et al which appears on Pages 339-343 of the 1982 Thin Solid Films (Volume 9) journal it is disclosed that the deposited layer of titanium was not very pure (copper and chlorine contaminants) and was porous. There is no disclosure in this publication of the need or desire to deposit a titanium silicide layer nor is there a disclosure or suggestion that any titanium alloy layer such as titanium silicide so deposited would be capable of annealing at a temperature as low as 600.degree. C. and would also be substantially non-porous and of relatively high purity and be more easily reproducible in high volume production.
Thus, a need has existed to provide an economical method for the low-temperature deposition and annealing of refractory metal silicides having low resistivity, high purity, relatively low porosity, good reproducibility and uniformity, and otherwise suitable for use in microelectronic applications.